SPL is not for “advanced debugging” but for initial boot. SPL is a cutdown version of u-boot that only has one job: find the u-boot partition on a device, that may or may not be supported by ROM.
The boot flow on RK3588 roughly goes:
ROM -> load IDB (DDR+SPL) -> run DDR -> run SPL -> find storage device with u-boot -> load u-boot -> boot OS.